Vaccinated People Also Spread the Delta Variant, Yearlong Study Shows

Study found similar peak viral load with or without shots

Immunized household contacts have a 25% chance of infection

People inoculated against Covid-19 are just as likely to spread the 
delta variant of the virus to contacts in their household as those 
who haven’t had shots, according to new research.

In a yearlong study of 621 people in the U.K. with mild Covid-19, 
scientists found that their peak viral load was similar regardless 
of vaccination status, according to a paper published Thursday in 
The Lancet Infectious Diseases medical journal. The analysis also 
found that 25% of vaccinated household contacts still contracted the 
disease from an index case, while 38% of those who hadn’t had shots 
became infected.

The results go some way toward explaining why the delta variant is 
so infectious even in nations with successful vaccine rollouts, and 
why the unvaccinated can’t assume they are protected because others 
have had shots. Those who were inoculated cleared the virus more 
quickly and had milder cases, while unvaccinated household members 
were more likely to suffer from severe disease and hospitalization.

“Our findings show that vaccination alone is not enough to prevent 
people from being infected with the delta variant and spreading it 
in household settings,” said Ajit Lalvani, a professor of infectious 
diseases at Imperial College London who co-led the study. “The 
ongoing transmission we are seeing between vaccinated people makes 
it essential for unvaccinated people to get vaccinated to protect 
themselves.”

Vaccination was found to reduce household transmission of the alpha 
variant -- first discovered in the U.K. in late 2020 -- by between 
40% and 50%, and infected vaccinated individuals had a lower viral 
load in the upper respiratory tract than those who hadn’t had shots. 
The delta variant has been the dominant strain globally for some 
time, however. 

The research also showed that immunity from full vaccination waned 
in as little as three months. The authors said there wasn’t enough 
data to advise on whether this should lead to a change in the U.K.’s 
booster policy, where third doses are currently being offered to 
older and more vulnerable people six months after their second shot.

Six months was an arbitrary time period chosen following early data 
from Israel on the effectiveness of boosters, but there is no reason 
to believe they would be less effective if given earlier, said Neil 
Ferguson, an epidemiologist at Imperial College London and 
investigator on the study, at a press briefing Thursday. 

The booster program could help halt the virus, as extra shots or 
repeated infections tend to lead to longer immunological memory, 
potentially protecting people for up to a year, Lalvani said. More 
data are needed to confirm this, he said.

The authors didn’t analyze infections based on the type of vaccines 
people had received. Maria Zambon, head of influenza and respiratory 
virology at the U.K. Health Security Agency, noted that there are 
still more than 300 vaccines in development, and said it’s possible 
that future generations of shots may be better at preventing 
transmission.

https://www.bloomberg.com/news/articles/2021-10-28/getting-
vaccinated-doesn-t-stop-people-from-spreading-delta